Question

I have a List<Person> and instead want to convert them for simple processing to a List<string>, doing the following:

List<Person> persons = GetPersonsBySeatOrder();
List<string> seatNames = persons.Select(x => x.Name).ToList();

Console.WriteLine("First in line: {0}", seatNames[0]);

Is the .Select() statement on a LINQ to Objects object guaranteed to not change the order of the list members? Assuming no explicit distinct/grouping/ordering is added

Also, if an arbitrary .Where() clause is used first, is it still guaranteed to keep the relative order, or does it sometimes use non-iterative filtering?

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As Fermin commented above, this is essentially a duplicate question. I failed on selecting the correct keywords to search stackoverflow

Preserving order with LINQ

Answer 1

In current .Net implementation it use such code. But there are no guarantee that this implementation will be in future.

private static IEnumerable<TResult> SelectIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, int, TResult> selector)
{
  int index = -1;
  foreach (TSource source1 in source)
  {
    checked { ++index; }
    yield return selector(source1, index);
  }
}